Composition for ceramic substrate, ceramic substrate, process for producing ceramic substrate and glass composition

ABSTRACT

A ceramic substrate composition is provided which can be co-fired with a low-melting metal and exhibits excellent dielectric characteristics at high frequencies, particularly tens of gigahertz. The ceramic substrate composition mainly contains a glass represented by aB 2 O 3 -bRe 2 O 3 -cZnO, wherein the molar ratios (a, b, and c) fall within a region defined in a ternary composition diagram by Point A (0.4, 0.595, 0.005), Point B (0.4, 0.25, 0.35), Point C (0.52, 0.01, 0.47), Point D (0.9, 0.005, 0.095), and Point E (0.9, 0.09, 0.01).

TECHNICAL FIELD

The present invention relates to glass-containing ceramic substratecompositions, ceramic substrates prepared with the compositions, andmethods for producing the ceramic substrates.

BACKGROUND ART

In recent years, information processors, typified by computers andmobile communication systems, have dramatically achieved higherinformation processing speeds, size reductions, and a larger number offunctions. Such improvements in the performance of informationprocessors have been realized mainly with increasing packing densities,speeds, and performance of semiconductor devices.

Semiconductor devices are conventionally mounted on alumina insulatingsubstrates, which are fired at 1,500° C. to 1,600° C. Accordingly,high-melting metals such as Mo, Mo—Ni, and W must be used as thematerial for patterned conductors to co-fire the insulating substrateswith the patterned conductors. Such high-melting metals, however, havehigh resistivity and thus pose difficulty in sufficiently exploiting theperformance of semiconductor devices for higher information processingspeeds and higher processing signal frequencies.

A variety of ceramic substrate compositions have been developed whichcan be co-fired with low-melting metals having low resistivity, such asAg and Cu. An example is a ceramic substrate composition prepared byadding Al₂O₃ to a CaO—SiO₂—Al₂O₃—B₂O₃-based glass (see Patent Document1). Patent Document 1: Japanese Examined Patent Application PublicationNo. 3-53269

DISCLOSURE OF INVENTION

Problems to be Solved by the Invention

The ceramic substrate composition disclosed in Patent Document 1 can befired at 800° C. to 1,000° C. Use of the ceramic substrate compositionallows the production of a ceramic substrate including internalpatterned conductors made of a low-melting metal such as Ag or Cu.

The ceramic substrate produced by firing the ceramic substratecomposition, however, has high dielectric loss and particularly, cannotbe used for the GHz range. Although the ceramic substrate can be usedtogether with patterned conductors made of metals with low resistivity,such as Ag and Cu, the high dielectric loss makes it difficult toincrease the processing speeds and processing signal frequencies ofinformation processors using the ceramic substrate.

An object of the present invention, which has been created under theabove circumstances, is to provide a ceramic substrate composition thatcan be co-fired with a low-melting metal and exhibits excellentcharacteristics at high frequencies, e.g., in the GHz range, and alsoprovide a ceramic substrate prepared with the ceramic substratecomposition and a method for producing the ceramic substrate.

Means for Solving the Problems

That is, the present invention provides a ceramic substrate compositioncontaining a glass composition represented by aB₂O₃-bRe₂O₃-cZnO (whereinRe is a rare earth element; and a+b+c=1 [wherein a, b, and c are molarratios]). The molar ratios (a, b, and c) fall within a region defined ina ternary composition diagram by Point A (0.4, 0.595, 0.005), Point B(0.4, 0.25, 0.35), Point C (0.52, 0.01, 0.47), Point D (0.9, 0.005,0.095), and Point E (0.9, 0.09, 0.01).

The present invention further provides a ceramic substrate includingReBO₃ and/or ReB₃O₆ precipitated as a main crystal phase. The ceramicsubstrate is produced by forming the above ceramic substrate compositionaccording to the present invention into a predetermined shape and firingthe shaped product.

The present invention further provides a method for producing a ceramicsubstrate. This method includes forming the above ceramic substratecomposition according to the present invention into a predeterminedshape and firing the shaped product.

Advantages

The ceramic substrate composition according to the present inventioncontains the glass represented by aB₂O₃-bRe₂O₃-cZnO (wherein Re is arare earth element; and a+b+c=1 [wherein a, b, and c are molar ratios]).The molar ratios (a, b, and c) fall within the region defined in theternary composition diagram by Point A (0.4, 0.595, 0.005), Point B(0.4, 0.25, 0.35), Point C (0.52, 0.01, 0.47), Point D (0.9, 0.005,0.095), and Point E (0.9, 0.09, 0.01). This ceramic substratecomposition can be co-fired with a low-melting metal such as silver orcopper, and can be used to provide a ceramic substrate having excellentcharacteristics at high frequencies, e.g., in the GHz range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a ternary composition diagram showing the composition range ofa glass represented by the general formula A.

FIG. 2 is a schematic sectional view of a ceramic substrate according toan embodiment of the present invention.

FIG. 3 is a schematic sectional view illustrating a step of a processfor producing a ceramic substrate shown in FIG. 2.

FIG. 4 is a graph showing measurements by XRD before and after thefiring of glass compositions according to the present invention.

FIG. 5 is a graph showing measurements by XRD after the firing of aceramic substrate composition according to the present invention.

REFERENCE NUMERALS

-   -   1 ceramic substrate    -   2 ceramic laminate    -   2 a, 2 b, 2 c, 2 d, and 2 e ceramic layer    -   5 internal line conductor    -   6 via hole conductor    -   7 and 8 external electrode    -   9 a and 9 b surface-mount component

BEST MODE FOR CARRYING OUT THE INVENTION

A ceramic substrate composition according to the present invention willnow be described.

The ceramic substrate composition according to the present inventioncontains a glass composition represented by the following generalformula A:aB₂O₃-bRe₂O₃-cZnO(wherein Re is a rare earth element; and a+b+c=1 [wherein a, b, and care molar ratios]). In a ternary composition diagram shown in FIG. 1,the molar ratios (a, b, and c) in the above general formula A fallwithin a region defined by:

-   -   Point A (0.4, 0.595, 0.005),    -   Point B (0.4, 0.25, 0.35),    -   Point C (0.52, 0.01, 0.47),    -   Point D (0.9, 0.005, 0.095), and    -   Point E (0.9, 0.09, 0.01)        (the above region includes molar ratios on the lines between        Points A, B, C, D, and E).

This glass is produced by melting and rapidly cooling a raw materialincluding a compound (preferably, an oxide) containing boron, a compound(preferably, an oxide) containing a rare earth element, and a compound(preferably, an oxide) containing zinc. The glass may be completelyamorphous or partially crystallized. Crystal phases (crystal compounds)such as ReBO₃, Re₃BO₆, and ReB₃O₆ (wherein Re is a rare earth element)can be precipitated after the firing of the glass by optimally selectingthe composition of the glass represented by the general formula A andthe compositions and amounts of additives added to the glass. A ceramicsubstrate produced with the ceramic substrate composition exhibitsextremely low dielectric loss, e.g., an fQ of 25,000 GHz or more (thatis, a Q of 2,500 or more at 10 GHz), excellent stability to temperature,e.g., a rate of change in resonance frequency with respect to change intemperature within ±50 ppm/° C., and high strength, e.g., a flexuralstrength of 150 MPa or more.

In the glass represented by the general formula A, B₂O₃ functions as anoxide that forms the network of the glass or a component of possiblecrystal phases. For the ceramic substrate composition according to thepresent invention, boron is contained in the glass, which is prepared bymelting and rapid cooling; therefore, boron oxide, which is generallyknown to have low resistance to humidity, can be subjected to thereaction by firing at low temperature. In addition, the firingtemperature for the ceramic substrate composition and the resistivity ofthe sintered ceramic product (ceramic substrate) vary depending on thetype of rare earth element contained in the glass. The firingtemperature and the resistivity can therefore be optionally controlledusing a desired combination of elements. Furthermore, the addition ofZnO allows stable production of the glass and the maintenance of lowdielectric loss.

If the content of B₂O₃ exceeds a molar ratio of 0.9 in the compositionregion shown in FIG. 1, B₂O₃ that does not contribute to theprecipitation of an ReBO₃—ZnO crystal phase partially liquefies andcauses high dielectric loss at high frequencies. If, on the other hand,the content of B₂O₃ falls below a molar ratio of 0.4, the glass cannotbe stably prepared, and thus the resultant ceramic substrate exhibitshigh dielectric loss. If the content of Re₂O₃ exceeds a molar ratio of0.595, the ceramic substrate composition has difficulty in being firedat a temperature at which a low-melting metal such as Ag or Cu can beco-fired. If, on the other hand, the content of Re₂O₃ falls below amolar ratio of 0.005, crystal compounds are difficult to precipitate,and thus the ceramic substrate composition has difficulty in providing aceramic substrate having low dielectric loss at high frequencies. If thecontent of ZnO exceeds a molar ratio of 0.47, the resultant ceramicsubstrate exhibits high dielectric loss at high frequencies. If, on theother hand, the content of ZnO falls below a molar ratio of 0.005, theglass cannot be stably prepared, and thus the resultant ceramicsubstrate exhibits high dielectric loss.

Various rare earth elements, particularly lanthanoids, may be used asthe rare earth element (Re) contained in the glass compositionrepresented by the general formula A. Examples of the rare earth elementused include lanthanum (La), neodymium (Nd), cerium (Ce), samarium (Sm),and terbium (Tb); among them, at least one rare earth element selectedfrom the group consisting of lanthanum and neodymium is preferably used.The ionic radius of the Re (rare earth element) contained in ReBO₃ isrelatively large; ReBO₃ very probably precipitates from the glassrepresented by the general formula A. Lanthanum and neodymium arepreferably used because they have an orthorhombic crystal structure andallow the production of a dense sintered ceramic material with a lowfiring temperature at which a low-melting metal such as Ag or Cu can beco-fired. These rare earth elements can also reduce the dielectric lossat high frequencies.

The ceramic substrate composition according to the present inventionpreferably further contains at least one oxide selected from the groupconsisting of aluminum oxide (Al₂O₃) and titanium oxide (TiO₂) in anamount of 50% or less by weight of the total amount of the ceramicsubstrate composition. The oxide, such as Al₂O₃ and TiO₂, is preferablycontained as a ceramic additive, though the oxide may also be containedin the raw material for preparation of the glass represented by thegeneral formula A. Such a specific content of the oxide in the ceramicsubstrate composition can improve the strength of the resultant ceramicsubstrate. If, particularly, aluminum oxide is mainly contained, theresultant ceramic substrate exhibits a resistivity of about 7 to 10. If,on the other hand, titanium oxide is mainly contained, the resistivityof the ceramic substrate can be increased to about 16. If the content ofthe oxide exceeds 50% by weight of the total amount of the ceramicsubstrate composition, the firing temperature for the ceramic substratemust be increased, and thus the ceramic substrate tends to havedifficulty in being fired at 1,000° C. or less.

The ceramic substrate composition according to the present invention mayfurther contain a mixed oxide ceramic material including at least onecrystal phase selected from the group consisting of ReBO₃, Re₃BO₆, andReB₃O₆ (wherein Re is a rare earth element). That is, the mixed oxideceramic material, which mainly includes ReBO₃, Re₃BO₆, or ReB₃O₆, may beadded to the glass represented by the general formula A as an additiveto produce a ceramic substrate having excellent dielectriccharacteristics at high frequencies with a low firing temperature. Themixed oxide ceramic material can be prepared by calcining a raw materialincluding a compound (preferably, an oxide) containing a rare earthelement and a compound (preferably, an oxide) containing boron at about800° C. to 1,300° C.

The mixed oxide ceramic material is preferably produced by calcining araw material represented by dB₂O₃-eRe₂O₃ (wherein Re is a rare earthelement; and d/e=1/3 to 3/1 [wherein d and e are molar ratios]). Thisrange of composition provides a ceramic substrate having low dielectricloss at high frequencies without significant deterioration of thecharacteristics of the glass. If the molar ratio d/e falls below 1/3,the ceramic substrate composition tends to have difficulty in beingfired at a temperature at which a low-melting metal such as Ag or Cu canbe co-fired. If, on the other hand, the molar ratio d/e exceeds 3/1, theceramic substrate composition may fail to provide a ceramic substratehaving a desired shape. The amount of B₂O₃ and Re₂O₃ added is preferablydetermined such that the total composition of the resultant ceramicsubstrate falls within the region in the ternary composition diagramshown in FIG. 1.

The mixed oxide ceramic material represented by dB₂O₃-eRe₂O₃ preferablyfurther contains ZnO. The ZnO contained in the mixed oxide ceramicmaterial can lower the firing temperature for the ceramic substratecomposition without significant deterioration of the characteristics ofthe resultant ceramic substrate. The content of ZnO in the mixed oxideceramic material is not particularly limited; preferably, the content isdetermined such that the total composition of the resultant ceramicsubstrate falls within the region in the ternary composition diagramshown in FIG. 1.

The content of the mixed oxide ceramic material in the ceramic substratecomposition is preferably 20% to 90% by weight of the total amount ofthe glass represented by the general formula A and the mixed oxideceramic material. If the content falls below 20% by weight of the totalamount of the glass and the mixed oxide ceramic material, the glass maymelt during the firing of the ceramic substrate composition and fail toprovide a ceramic substrate having a desired shape. If, on the otherhand, the content exceeds 90% by weight of the total amount of the glassand the mixed oxide ceramic material, the ceramic substrate compositiontends to have difficulty in being fired at a temperature at which alow-melting metal such as Ag or Cu can be co-fired.

The ceramic substrate composition according to the present invention mayfurther contain a crystallization glass that can precipitate at leastone crystal phase selected from the group consisting of ReBO₃, Re₃BO₆,and ReB₃O₆ (wherein Re is a rare earth element). That is, thecrystallization glass, which can precipitate the above mixed oxidecrystal phases, may be added to the glass represented by the generalformula A as an additive to lower the firing temperature for the ceramicsubstrate composition while maintaining the characteristics of theresultant ceramic substrate. In particular, an ReBO₃—ZnO crystal phasemay melt at 850° C. to 900° C. when the ceramic substrate compositioncontains a large amount of glass represented by the general formula A ordepending on the composition and amount of mixed oxide ceramic materialadded to the glass. In that case, the addition of the crystallizationglass allows stable production of a ceramic substrate having lowporosity and low dielectric loss at high frequencies. Thecrystallization glass, which is prepared by melting and rapidly coolinga raw material, refers to a “glass partially including a crystal phase”or a “glass having the ability to precipitate a crystal phase infiring.”

Preferably, the crystallization glass is represented bygB₂O₃-hRe₂O₃-iZnO (wherein Re is a rare earth element; and g+h+i=1[wherein g, h, and i are molar ratios]), and the molar ratios (g, h, andi) fall within a region defined in a ternary composition diagram byPoint F (0.4, 0.595, 0.005), Point G (0.4, 0.25, 0.35), Point H (0.52,0.01, 0.47), Point I (0.9, 0.005, 0.095), and Point J (0.9, 0.09, 0.01).This region covers the same composition range as that of the glassrepresented by the general formula A. If the crystallization glass hassuch a composition, the ceramic substrate composition can be fired at atemperature at which a low-melting metal such as Ag or Cu can beco-fired. The ceramic substrate composition therefore allows stableproduction of a ceramic substrate having low dielectric loss at highfrequencies, e.g., several to tens of gigahertz. The composition of thecrystallization glass is particularly preferably selected such that thetotal composition of the resultant ceramic substrate falls within theregion in the ternary composition diagram shown in FIG. 1.

The content of the crystallization glass is preferably 20% to 90% byweight of the total amount of the glass represented by the generalformula A and the crystallization glass. If the content of thecrystallization glass falls below 20% by weight of the total amount ofthe glass represented by the general formula A and the mixed oxideceramic material, the glass may melt during the firing of the ceramicsubstrate composition and fail to provide a ceramic substrate having adesired shape. If, on the other hand, the content of the crystallizationglass exceeds 90% by weight of the total amount of the glass representedby the general formula A and the crystallization glass, the ceramicsubstrate composition tends to have difficulty in being fired at atemperature at which a low-melting metal such as Ag or Cu can beco-fired.

The ceramic substrate composition according to the present invention mayfurther contain a rare earth oxide represented by Re₂O₃ (wherein Re is arare earth element) In particular, when a ceramic substrate compositioncontaining a large absolute amount of B₂O₃ is co-fired with Ag, theceramic substrate composition may readily react with Ag. Addition ofRe₂O₃, however, allows stable production of a ceramic substrate havinglow dielectric loss at high frequencies. The rare earth elementcontained is preferably at least one element selected from the groupconsisting of lanthanum and neodymium.

Next, a ceramic substrate according to the present invention isdescribed below with reference to FIG. 2.

A ceramic substrate 1 includes a ceramic laminate 2 having a multilayerstructure of ceramic layers 2 a, 2 b, 2 c, 2 d, and 2 e made of theceramic substrate composition according to the present invention.

The ceramic laminate 2 has internal patterned conductors and externalpatterned conductors formed on the ceramic layers 2 a, 2 b, 2 c, 2 d,and 2 e. The internal patterned conductors include internal lineconductors 5 formed on the interfaces between the ceramic layers 2 andvia hole conductors 6 penetrating the ceramic layers. The externalpatterned conductors include external electrodes 8 formed on a mainsurface 3 of the ceramic laminate 2 and external electrodes 7 formed onthe other main surface 4 of the ceramic laminate 2. The externalelectrodes 7 are used as land electrodes for connecting the ceramicsubstrate 1 to a mother board (not shown). The external electrodes 8 areused for connection to surface-mount components 9 a and 9 b mounted onthe surface of the ceramic laminate 2. The surface-mount component 9 ais, for example, a monolithic ceramic capacitor chip having planarterminal electrodes, and the surface-mount component 9 b is, forexample, a semiconductor device having bump electrodes.

The patterned conductors mainly contain at least one metal selected fromthe group consisting of Ag and Cu. Low-melting metals such as Ag and Cuare particularly preferred because they have lower resistivity thanhigh-melting metals such as tungsten and molybdenum and exhibitexcellent electrical conductivity at high frequencies.

The glass matrix of the ceramic substrate precipitates at least onemixed oxide crystal phase (namely, a crystal phase of a mixed oxidecontaining a rare earth element and boron) selected from the groupconsisting of ReBO₃, ReB₃O₆ (or Re(B₂O₃)₃), and Re₃BO₆. If the rawmaterial contains Al₂O₃, the glass matrix can further precipitatecrystal phases such as ReAl₃(BO₃)O₄, ReAl₃(BO₃)₄, and Al₂O₃. If, on theother hand, the raw material contains TiO₂, the glass matrix can furtherprecipitate crystal phases such as TiO₂. Such crystal phases contributesignificantly to, for example, the reduction in dielectric loss at highfrequencies, the improvement in the strength of the sintered laminate,and the control of relative dielectric constant.

Next, a method for producing the ceramic substrate 1 shown in FIG. 2 isdescribed below with reference to FIG. 3.

The ceramic laminate 2 included in the ceramic substrate 1 shown in FIG.2 is prepared by firing a composite laminate 11 shown in FIG. 3.

The composite laminate 11 includes an unfired ceramic laminate 2′prepared by laminating ceramic green sheets 2 a′, 2 b′, 2 c′, 2 d′, and2 e′ corresponding to the ceramic layers 2 a, 2 b, 2 c, 2 d, and 2 e,respectively, and constraining layers 12 provided on the main surfaces 3and 4 of the unfired ceramic laminate 2′. That is, the unfired ceramiclaminate 2′ is a laminate of unfired ceramic layers (ceramic greensheets) made of the ceramic substrate composition according to thepresent invention. The constraining layers 12 contain a ceramic materialthat does not sinter at the sintering temperature of the ceramicsubstrate composition according to the present invention. The ceramicmaterial used may be, for example, alumina powder.

In addition, various patterned conductors are provided on the ceramicgreen sheets of the unfired ceramic laminate 2′. The patternedconductors include unfired internal line conductors 5′ that are fired toform the internal line conductors 5, unfired via hole conductors 6′ thatare fired to form the via hole conductors 6, and unfired externalelectrodes 7 and 8 that are fired to form the external electrodes 7 and8, respectively.

The unfired composite laminate 11 is prepared by, for example, thefollowing process.

Appropriate amounts of additives are added to a powder of a glassrepresented by the general formula A. The resultant mixed powder ismixed with 10% to 50% by weight of a binder such as a butyral binder oran acrylic binder and, optionally, appropriate amounts of a plasticizersuch as phthalic acid and an organic solvent such as isopropyl alcoholor toluene to prepare a ceramic slurry for preparation of the ceramicgreen sheets 2 a′, 2 b′, 2 c′, 2 d′, and 2 e′. This slurry is formedinto sheets with, for example, a doctor blade to prepare ceramic greensheets. These ceramic green sheets are optionally perforated to form viaholes in which the via hole conductors 6 are formed. These via holes arefilled with a Ag- or Cu-based conductive paste or powder to form theunfired via hole conductors 6′. In addition, the unfired externalelectrodes 7′ and 8′ and the unfired internal line conductors 5′ areoptionally formed on the ceramic green sheets by screen printing with aAg- or Cu-based conductive paste or powder. The ceramic green sheetsthus formed are laminated in a predetermined order to prepare theunfired ceramic laminate 2′.

On the other hand, a ceramic powder, such as alumina powder, is mixedwith appropriate amounts of a binder, a dispersant, a plasticizer, andan organic solvent as described above to prepare a ceramic slurry forpreparation of ceramic green sheets for the constraining layers 12. Thisslurry is formed into sheets with, for example, a doctor blade toprepare ceramic green sheets for the constraining layers 12.

A predetermined number of the ceramic green sheets for the constraininglayers 12 are laminated on the top and bottom of the unfired ceramiclaminate 2′, which is, for example, pressed at 30 to 100 kgf/cm² afterpreheating at 50° C. to 100° C. to prepare the composite laminate 11. Asshown in FIG. 3, the composite laminate 11 includes the unfired ceramiclaminate 2′, and the constraining layers 12 are bonded to the mainsurfaces thereof. The composite laminate 11 may optionally be cut to anappropriate size. The ceramic green sheets for the constraining layers12, which are bonded to the top and bottom main surfaces of the unfiredceramic laminate 2′, may also be bonded to either the main surface 3 orthe main surface 4.

The composite laminate 11 is then fired at 1,000° C. or less,particularly about 800° C. to 1,000° C. The firing process is carriedout in an oxidizing atmosphere, such as air, for Ag-based patternedconductors and in a reducing atmosphere, such as N₂, for Cu-basedpatterned conductors. The composite laminate 11 may be fired with apredetermined pressure being applied thereto in the vertical directionor with no pressure being applied.

In the firing step, the constraining layers 12 themselves do notsubstantially sinter or shrink; therefore, they exert a constraint forceon the ceramic laminate 2′ to constrain the shrinkage thereof in a planeduring the firing. As the ceramic substrate composition contained in theceramic laminate 2 sinters, the ceramic laminate 2 shrinks substantiallyonly across the thickness with the shrinkage in the plane beingconstrained. After the constraining layers are removed by, for example,wet blasting or brushing, a ceramic substrate having excellent surfaceflatness and dimensional accuracy in the plane can be provided.Subsequently, surface-mount components such as passive components, e.g.,monolithic ceramic capacitor chips, and active components, e.g.,semiconductor devices, are optionally mounted on the ceramic substrateto produce the ceramic substrate 1 shown in FIG. 2.

It is generally required in a process using constraining layers asdescribed above that the constraining layers do not react with a ceramicsubstrate composition during the firing and can readily be removed afterthe firing. The ceramic substrate composition according to the presentinvention can be fired in the above process because the composition doesnot substantially react with the constraining layers, for examplealumina layers. The ceramic substrate provided after the firingtherefore has excellent dimensional accuracy, namely a shrinkage of0.05% or less in the X and Y directions.

The ceramic substrate and the method for producing the ceramic substrateaccording to an embodiment of the present invention have been described,though ceramic substrates and methods for producing the ceramicsubstrates according to the present invention are not limited to theabove embodiment. The present invention may be applied to, for example,a functional substrate that has various surface-mount components mountedon a main surface thereof, as described above, and that incorporates aninductor, a capacitor, or a resistor. The present invention may also beapplied to, for example, a substrate for a single-function componentwith no surface-mount components mounted thereon.

In addition, the ceramic substrate composition according to the presentinvention may be applied not only to the above process usingconstraining layers, but also to normal processes, that is, processeswithout constraining layers.

The ceramic substrate according to the present invention, which has lessdielectric loss at high frequencies and excellent high-frequencycharacteristics, is suitable for, for example, electronic componentsdesigned for microwaves and millimeter waves, including car phones,radio equipment for business and domestic uses, and cell phones.

EXAMPLES

The present invention is described with specific examples below.

Examination of Crystallization By Firing

Glass compositions according to the present invention were examined forcrystallization by firing. FIG. 4 shows measurements by XRD before andafter the firing of glasses A and B. The glass A is an amorphous glasscontaining B₂O₃, La₂O₃, and ZnO at a ratio of 3.0:1.0:0.5=0.67:0.22:0.11(in terms of molar ratio). In FIG. 4, line (d) indicates theidentification data of the glass A before firing, and line (c) indicatesthe data after firing in air at 900° C. for two hours. The results inFIG. 4 clearly show that LaB₃O₆ precipitated as a main crystal phaseafter the firing though no crystal phase was found before the firing.

The glass B is a crystallized glass containing B₂O₃, La₂O₃, and ZnO at aratio of 2.0:1.0:0.5=0.57:0.29:0.14 (in terms of molar ratio). In FIG.4, line (b) indicates the identification data of the glass B beforefiring, and line (a) indicates the data after firing in air at 900° C.for two hours. The results in FIG. 4 clearly show that LaBO₃ wasobserved as a main crystal phase after the firing more clearly thanbefore the firing.

The results also show that LaB₃O₆ tends to precipitate as a main crystalphase for a relatively high content of boron, as in the glass A, whileLaBO₃ tends to precipitate as a main crystal phase for a relatively lowcontent of boron, as in the glass B. The measurement by XRD was carriedout using a vertical 2-axis goniometer with a copper vessel at a vesselvoltage of 40 kV, a vessel current of 40 mA, and a scanning speed of2.000°/min.

FIG. 5 shows measurements by XRD for a ceramic substrate compositionaccording to the present invention which contains 20% by weight of theamorphous glass A and 80% by weight of the crystallized glass B. Theceramic substrate composition was fired (a) at 900° C. for two hours,(b) at 875° C. for two hours, (c) at 850° C. for two hours, (d) at 850°C. for 30 minutes, and (e) at 825° C. for two hours, as shown in FIG. 5.

The results in FIG. 5 clearly show that LaB₃O₆ and LaBO₃ precipitated asmain crystal phases at a firing temperature of 825° C. or more. Inparticular, the main crystal phases were clearly found at firingtemperatures above 875° C. While ceramic substrate compositions aregenerally fired at about 1,200° C., the ceramic substrate compositionaccording to the present invention started to precipitate the crystalphases at about 675° C. Ceramic substrates including LaB₃O₆ and LaBO₃crystal phases having satisfactory characteristics were provided at 725°C. or more, particularly 800° C. or more. These results show that LaB₃O₆and LaBO₃ crystal phases appear at relatively low temperatures, namelyabout 725° C. to 900° C., to provide a ceramic substrate havingexcellent high-frequency characteristics.

Preparation of Glass

La₂O₃ powder, B₂O₃ powder, and ZnO powder were mixed at ratios shown inTables 1 to 4 below to prepare raw materials for glasses. These mixtureswere placed in a platinum crucible, were melted at about 1,200° C., andwere rapidly cooled to prepare glasses. These glasses were pulverized toa median size (D50) of 20 μm or less to prepare glass powders as mainraw materials.

Preparation of Ceramic Material

La₂O₃ powder, B₂O₃ powder, and ZnO powder were mixed at ratios shown inTables 1 to 4 below to prepare raw materials for ceramic materials. Purewater or ethanol was then added to the mixtures, which were wet-mixed ina ball mill with zirconia balls or alumina balls for 10 to 30 hours. Theresultant mixtures were dried, were calcined at 1,000° C., and werepulverized to a median size of 10 μm or less to prepare ceramic powders(calcined powders).

Analysis by powder X-ray diffraction identified the presence of any oneof LaBO₃, La₃BO₆, and LaB₃O₆ crystal phases in the calcined powderscontaining boron and lanthanum at a ratio of B/La=1/3 to 3/1. Theanalysis also identified the presence of any one of NdBO₃, Nd₃BO₆, andNdB₃O₆ crystal phases in the calcined powders containing boron andneodymium at a ratio of B/Nd=1/3 to 3/1.

Preparation of Crystallization Glass

La₂O₃, B₂O₃, and ZnO were mixed at ratios shown in Tables 1 to 4 belowto prepare raw materials for crystallization glasses. These mixtureswere placed in a platinum crucible, were melted at about 1,250° C., andwere rapidly cooled to prepare crystallization glasses. These glasseswere pulverized to a median size of 20 μm or less to preparecrystallization glass powders.

Analysis by powder X-ray diffraction identified the presence of any oneof LaBO₃, La₃BO₆, and LaB₃O₆ crystal phases in the crystallization glasspowders represented by gB₂O₃-hLa₂O₃-iZnO wherein the molar ratios (g, h,and i) fell within the region defined in a ternary composition diagramby Point F (0.4, 0.595, 0.005), Point G (0.4, 0.25, 0.35), Point H(0.52, 0.01, 0.47), Point I (0.9, 0.005, 0.095), and Point J (0.9, 0.09,0.01). The analysis also identified the presence of any one of NdBO₃,Nd₃BO₆, and NdB₃O₆ crystal phases in the crystallization glass powdersrepresented by gB₂O₃-hNd₂O₃-iZnO wherein the molar ratios (g, h, and i)fell within the region defined in a ternary composition diagram by PointF (0.4, 0.595, 0.005), Point G (0.4, 0.25, 0.35), Point H (0.52, 0.01,0.47), Point I (0.9, 0.005, 0.095), and Point J (0.9, 0.09, 0.01).

Preparation of Ceramic Substrate Composition and Ceramic Substrate

The glass powders, ceramic powders (calcined powders), andcrystallization glass powders thus prepared were weighed out accordingto weight percentages shown in Tables 1 to 4 and were mixed. Otheradditives such as Al₂O₃ powder, TiO₂ powder, and La₂O₃ powder wereoptionally added to the resultant mixtures. Pure water or ethanol wasthen added to the mixtures, which were wet-mixed in a ball mill withzirconia balls or alumina balls for 10 to 30 hours. After the resultantmixtures were dried, an organic binder for molding was added to themixtures. These mixed powders were sized and molded into cylinders witha diameter of 15 mm and a height of 7.5 mm using a mold press. Thesecompacts were debinded at 500° C. to 600° C. in air and were fired atfiring temperatures shown in Tables 1 to 4. After the firing, theresultant sintered ceramic compacts were polished so that the surfacesthereof became smooth and the top and bottom surfaces thereof becameparallel. The sintered ceramic compacts were then washed with anultrasonic cleaner and were dried to prepare samples for evaluation ofdielectric characteristics (Sample No. 1 to 54). The evaluations of theresultant ceramic samples are shown in Tables 1 to 4.

The values of relative dielectric constant εr and fQ, which indicates aquality factor at high frequencies, were measured at 10 to 20 GHz usinga dielectric resonator with short-circuited terminals. The values of Tf,which indicates resonant frequency characteristics relative totemperature, were the averages of rates of change in resonant frequencyat −25° C. to +25° C. and +25° C. to +85° C. The resonant frequency wasmeasured using a resonator with short-circuited terminals by placing theceramic samples and a jig in a constant-temperature bath at apredetermined temperature. The flexural strength was measured by athree-point bending test. In this test, a ceramic sample having a widthof about 10 mm, a length of about 60 mm, and a thickness of about 1 mmwas placed on two fulcra 30 mm apart to determine the flexural strengthfrom the strength at the time when the sample fractured and thedimensions of the sample according to JIS R1601:1981. For the samplesprepared with the ceramic powders including a LaBO₃, La₃BO₆, or LaB₃O₆crystal phase, any one of LaBO₃, La₃BO₆, and LaB₃O₆ crystal phases wasalso observed in the sintered ceramic compacts. For the samples preparedwith the ceramic powders including a NdBO₃, Nd₃BO₆, or NdB₃O₆ crystalphase, on the other hand, any one of NdBO₃, Nd₃BO₆, and NdB₃O₆ crystalphases was observed in the sintered ceramic compacts. TABLE 1 FiringCharacteristics Additive temper- Flexural Glass Type/ Al₂O₃/ aturePorosity fQ T_(f) strength No. wt. % La₂O₃ B₂O₃ ZnO wt. % La₂O₃ B₂O₃ ZnOmaterial TiO₂ (° C.) (%) εr (GHz) (ppm/° C.) (MPa) 1 20 1 3 0.5 80 1 2 0Calcined 0 925 0.9 12.4 26,000 −47 175 powder 2 50 1 3 0.5 50 1 2 0Calcined 0 900 0.5 9.0 37,000 −45 180 powder 3 80 1 3 0.5 20 1 2 0Calcined 0 900 0.1 8.0 45,000 −42 190 powder 4 85 1 3 0.5 15 1 2 0Calcined 0 850 1.9 7.4 29,000 −55 200 powder 5 8 1 2 0.5 92 1 2 0Calcined 0 950 5.1 12.5 20,000 −51 150 powder 6 20 1 2 0.5 80 1 2 0Calcined 0 900 1.7 12.9 27,000 −48 140 powder 7 50 1 2 0.5 50 1 2 0Calcined 0 875 0.7 10.0 30,000 −47 140 powder 8 80 1 2 0.5 20 1 2 0Calcined 0 875 0.4 9.5 33,000 −46 150 powder 9 20 1 2 1 80 1 2 0Calcined 0 950 0.5 12.1 26,000 −47 150 powder 10 50 1 2 1 50 1 2 0Calcined 0 900 0.6 11 29,000 −40 160 powder 11 80 1 2 1 20 1 2 0Calcined 0 875 0.5 10 34,000 −45 150 powder 12 20 1 4 2 80 1 2 0Calcined 0 875 0.6 10.5 25,000 −45 160 powder 13 50 1 4 2 50 1 2 0Calcined 0 900 0.5 10 39,000 −40 170 powder 14 80 1 4 2 20 1 2 0Calcined 0 850 0.1 9.8 48,000 −41 180 powder 15 20 1 2 0.1 80 1 2 0Calcined 0 925 0.2 9.9 39,000 −40 190 powder 16 50 1 2 0.1 50 1 2 0Calcined 0 900 0.1 9.0 46,000 −41 200 powder 17 80 1 2 0.1 20 1 2 0Calcined 0 850 0.1 7.4 49,000 −35 210 powder

TABLE 2 Firing Characteristics Additive temper- Flexural Glass Type/Al₂O₃/ ature Porosity fQ T_(f) strength No. wt. % La₂O₃ B₂O₃ ZnO wt. %La₂O₃ B₂O₃ ZnO material TiO₂ (° C.) (%) εr (GHz) (ppm/° C.) (MPa) 18 501 0 1 50 1 2 0 Calcined 0 1,050 0.5 13.1 6,200 −55 160 powder 19 50 0 11 50 1 2 0 Calcined 0 825 1 13 12,000 −61 180 powder 20 20 1 3 0.5 80 13 0 Calcined 0 925 0.5 10.5 40,000 −40 200 powder 21 50 1 3 0.5 50 1 4 0Calcined 0 800 3.5 7.1 18,000 −49 145 powder 22 50 1 3 0.5 50 1 1 1Calcined 0 900 0.4 10 31,000 −41 170 powder 23 20 1 3 0.5 80 2 1 0Calcined 0 950 0.5 11.5 26,000 −41 180 powder 24 50 1 3 0.5 50 2 1 0Calcined 0 900 0.9 12.5 30,000 −40 200 powder 25 50 1 3 0.5 50 3 1 0Calcined 0 950 0.5 11 31,000 −41 150 powder 26 50 1 3 0.5 50 1 3 0.5Calcined 0 850 0.3 12.0 34,000 −45 150 powder 27 50 1 3 0.5 50 1 3 0.5Calcined 50(TiO₂₎ 875 0.1 15.6 37,000 +49 170 powder 28 50 1 3 0.5 50 13 0.5 Calcined 30(TiO₂₎ 900 0.2 14.1 31,000 +32 190 powder 29 50 1 3 0.550 1 3 0.5 Calcined 10(TiO₂₎ 900 0.1 10.1 35,000 +8  180 powder 30 50 13 0.5 50 1 3 0.5 Calcined  5(TiO₂₎ 875 0.2 9.5 31,000 −29 190 powder 3150 1 3 0.5 50 1 3 0.5 Calcined 30(Al₂O₃) 925 0.3 8.0 26,000 −39 260powder 32 50 1 3 0.5 50 1 3 0.5 Calcined  5(Al₂O₃) 925 0.2 7.9 31,000−32 280 powder 33 50 1 3 0.5 50 1 3 0.5 Calcined 20(Al₂O₃) 900 0.2 9.833,000 −25 220 powder 20(TiO₂) 34 20 0.9 8.5 0.5 80 1 1 0 Calcined 0 9250.1 7.5 51,000 −39 200 powder 35 90 1 3 0.5 10 1 2 0.5 Crystal- 0 8500.1 7.2 55,000 −42 180 lization glass 36 80 1 3 0.5 20 1 2 0.5 Crystal-0 875 0.1 7.5 77,000 −40 190 lization glass

TABLE 3 Firing Characteristics Additive temper- Flexural Glass Type/Al₂O₃/ ature Porosity fQ T_(f) strength No. wt. % La₂O₃ B₂O₃ ZnO wt. %La₂O₃ B₂O₃ ZnO material TiO₂ (° C.) (%) εr (GHz) (ppm/° C.) (MPa) 37 701 3 0.5 30 1 2 0.5 Crystal- 0 875 0 7.7 69,000 −41 200 lization glass 3860 1 3 0.5 40 1 2 0.5 Crystal- 0 900 0.1 7.9 72,000 −42 210 lizationglass 39 50 1 3 0.5 50 1 2 0.5 Crystal- 0 900 0 8.0 69,000 −41 200lization glass 40 40 1 3 0.5 60 1 2 0.5 Crystal- 0 900 0 7.9 71,000 −43180 lization glass 41 80 1 3 0.5 20 1 2 0.5 Crystal- 10(TiO₂) 850 0 7.856,000 +3  190 lization glass 42 50 1 3 0.5 50 1 2 0.5 Crystal- 10(TiO₂)875 0 8.0 59,000 −1  180 lization glass 43 50 1 3 0.5 50 1 2 0.5Crystal- 5(Al₂O₃) 900 0 7.7 39,000 −40 220 lization glass 44 90 1 4 0.510 1 0 0 La₂O₃ 0 900 0 8.1 54,000 −35 190 45 70 1 4 0.5 30 1 0 0 La₂O₃ 0900 0.2 7.9 57,000 −39 200 46 50 1 4 0.5 50 1 0 0 La₂O₃ 0 950 1 7.221,000 −48 180

TABLE 4 Firing Characteristics Additive temper- Flexural Glass Type/Al₂O₃/ ature Porosity fQ T_(f) strength No. wt. % La₂O₃ B₂O₃ ZnO wt. %La₂O₃ B₂O₃ ZnO material TiO₂ (° C.) (%) εr (GHz) (ppm/° C.) (MPa) 47 201 3 0.5 80 1 2 0 Calcined 0 950 0.9 13.6 25,000 −40 165 powder 48 50 1 30.5 50 1 2 0 Calcined 0 925 0.3 9.5 37,000 −45 170 powder 49 80 1 3 0.520 1 2 0 Calcined 0 900 0.1 9.0 30,000 −42 150 powder 50 85 1 3 0.5 15 12 0 Calcined 0 850 1.9 8.4 15,000 −53 180 powder 51 50 1 3 0.5 50 1 2 0Crystal- 0 900 0.3 9.6 45,000 −40 160 lization glass 52 50 1 3 0.5 50 12 0 Crystal- 10(TiO₂) 900 0.3 10.6 29,000 +10 160 lization glass 53 50 13 0.5 50 1 2 0 Crystal- 20(TiO₂) 900 0.2 11.5 27,000 +23 150 lizationglass 54 50 1 3 0.5 50 1 2 0 Crystal- 10(Al₂O₃) 925 0.1 8.5 25,000 −43200 lization glass

As shown above, the ceramic substrate compositions could be fired at1,000° C. or less, at which a low-melting metal such as silver or coppercan be co-fired, when the ceramic substrate compositions contained theglasses represented by the general formula A: aB₂O₃-bRe₂O₃-cZnO (whereinRe is lanthanum or neodymium; and a+b+c=1 [wherein a, b, and c are molarratios]); and when the molar ratios (a, b, and c) fell within the regiondefined by Point A (0.4, 0.595, 0.005), Point B (0.4, 0.25, 0.35), PointC (0.52, 0.01, 0.47), Point D (0.9, 0.005, 0.095), and Point E (0.9,0.09, 0.01) in the ternary composition diagram shown in FIG. 1.Furthermore, sintered ceramic compacts could be achieved which had lowdielectric loss at high frequencies, namely an fQ of 15,000 GHz or more,particularly 25,000 GHz or more.

In addition, the flexural strength of the resultant sintered ceramiccompacts could be significantly improved when the ceramic substratecompositions contained Al₂O₃ in an amount of 50% or less by weight ofthe total amount. On the other hand, the relative dielectric constant ofthe resultant sintered ceramic compacts could be increased when theceramic substrate compositions contained TiO₂ in an amount of 50% orless by weight of the total amount. Furthermore, the balance between theflexural strength and relative dielectric constant could be suitablycontrolled by adjusting the contents of these ceramic materials.

In addition, the firing temperatures for the ceramic substratecompositions could be decreased and the fQ of the resultant sinteredceramic compacts could be increased to, for example, 30,000 GHz or morewhen the ceramic substrate compositions contained at least one mixedoxide ceramic material selected from the group consisting of ReBO₃,Re₃BO₆, and ReB₃O₆ (wherein Re is lanthanum or neodymium) in an amountof 20% to 90% by weight of the total amount of the glass represented bythe general formula A. The fQ of the sintered ceramic compacts could befurther increased when the ceramic substrate compositions also containedZnO. Furthermore, sintered ceramic compacts could be achieved which hada flexural strength of 150 MPa or more with rates of change in resonantfrequency relative to temperature, Tf, within ±50 ppm/° C.

In particular, sintered ceramic compacts having a porosity of 0.3% orless and an fQ of 50,000 GHz or more could be achieved. This is becausethe ceramic substrate compositions contained a crystallization glassrepresented by gB₂O₃-hRe₂O₃-iZnO (wherein Re is lanthanum or neodymium;and g+h+i=1 [wherein g, h, and i are molar ratios]) in an amount of 20%to 90% by weight of the total amount of the glass represented by thegeneral formula A, and the molar ratios (g, h, and i) fell within theregion defined in a ternary composition diagram by Point F (0.4, 0.595,0.005), Point G (0.4, 0.25, 0.35), Point H (0.52, 0.01, 0.47), Point I(0.9, 0.005, 0.095), and Point J (0.9, 0.09, 0.01). The crystallizationglass could precipitate at least one mixed oxide selected from the groupconsisting of ReBO₃, Re₃BO₆, and ReB₃O₆ (wherein Re is lanthanum orneodymium). Furthermore, sintered ceramic compacts could be achievedwhich had a flexural strength of 150 MPa or more with rates of change inresonant frequency relative to temperature, Tf, within ±50 ppm/° C.

In addition, the sintered ceramic compacts could have an fQ of 50,000GHz or more when the ceramic substrate compositions contained La₂O₃ suchthat the total compositions of the sintered ceramic compacts fell withinthe region in the ternary composition diagram shown in FIG. 1.Furthermore, sintered ceramic compacts could be achieved which had aflexural strength of 150 MPa or more with rates of change in resonantfrequency relative to temperature, Tf, within ±50 ppm/° C.

1. A ceramic substrate composition comprising a glass represented byaB₂O₃-bRe₂O₃-cZnO in which Re is a rare earth element and the molaramounts a+b+c=1; and wherein the molar amounts (a, b, and c) fall withina region defined in a ternary composition diagram by Point A (0.4,0.595, 0.005), Point B (0.4, 0.25, 0.35), Point C (0.52, 0.01, 0.47),Point D (0.9, 0.005, 0.095), and Point E (0.9, 0.09, 0.01) or on thelines connection the Points.
 2. The ceramic substrate compositionaccording to claim 1, wherein the rare earth element is at least oneelement selected from the group consisting of lanthanum and neodymium.3. The ceramic substrate composition according to claim 1, furthercomprising at least one oxide selected from the group consisting ofaluminum oxide and titanium oxide in an amount of 50% or less by weightof the total amount.
 4. The ceramic substrate composition according toclaim 1, further comprising a mixed oxide ceramic material including atleast one crystal phase selected from the group consisting of ReBO₃,Re₃BO₆, Re(B₂O₃)₃ and ReB₃O₆.
 5. The ceramic substrate compositionaccording to claim 4, wherein the mixed oxide ceramic material iscalcined dB₂O₃-eRe₂O₃ in which the molar ratio d/e=1/3 to 3/1.
 6. Theceramic substrate composition according to claim 5, wherein the mixedoxide ceramic material further contains ZnO.
 7. The ceramic substratecomposition according to claim 4, wherein the content of the mixed oxideceramic material is 20% to 90% by weight of the total amount of theglass and the mixed oxide ceramic material.
 8. The ceramic substratecomposition according to claim 1, further comprising a crystallizationglass that can precipitate at least one crystal phase selected from thegroup consisting of ReBO₃, Re₃BO₆, Re(B₂O₃)₂ and ReB₃O₆.
 9. The ceramicsubstrate composition according to claim 8, wherein the crystallizationglass is represented by gB₂O₃-hRe₂O₃-iZnO in which g+h+i=1 and the molaramounts g, h, and i fall within a region defined in a ternarycomposition diagram by Point F (0.4, 0.595, 0.005), Point G (0.4, 0.25,0.35), Point H (0.52, 0.01, 0.47), Point I (0.9, 0.005, 0.095), andPoint J (0.9, 0.09, 0.01) or on the lines connected those Points. 10.The ceramic substrate composition according to claim 8, wherein thecontent of the crystallization glass is 20% to 90% by weight of thetotal amount of the glass and the crystallization glass.
 11. The ceramicsubstrate composition according to claim 1, further comprising a rareearth oxide comprising Re₂O₃.
 12. A ceramic substrate comprisingprecipitated ReBO₃ and/or ReB₃O₆ as a main crystal phase, the ceramicsubstrate being produced by the process of claim
 15. 13. The ceramicsubstrate according to claim 12, wherein the shaped product fired is alaminate of comprising a plurality of layers comprising the glass. 14.The ceramic substrate according to claim 13, having a patternedconductor comprising at least one metal selected from the groupconsisting of silver and copper on a layer.
 15. A method for producing aceramic substrate, comprising providing a predetermined shaped ceramicsubstrate composition comprising a glass represented byaB₂O₃-bRe₂O₃-cZnO in which Re is a rare earth element, a+b+c=1, and themolar amounts a, b, and c fall within a region defined in a ternarycomposition diagram by Point A (0.4, 0.595, 0.005), Point B (0.4, 0.25,0.35), Point C (0.52, 0.01, 0.47), Point D (0.9, 0.005, 0.095), andPoint E (0.9, 0.09, 0.01) or on the lines connecting those Points; andfiring the shaped ceramic substrate composition.
 16. The method forproducing a ceramic substrate according to claim 15, wherein the shapedproduct fired is a laminate of comprising a plurality of layerscomprising the glass.
 17. The method for producing a ceramic substrateaccording to claim 16, further comprising forming a patterned conductorcomprising at least one metal selected from the group consisting ofsilver and copper on at least one of the layers.
 18. The method forproducing a ceramic substrate according to claim 15, wherein the shapedproduct is fired at 1,000° C. or less.
 19. (canceled)
 20. A glasscomposition represented by aB₂O₃-bRe₂O₃-cZnO in which Re is a rare earthelement; a+b+c=1; and the molar amounts a, b, and c fall within a regiondefined in a ternary composition diagram by Point A (0.4, 0.595, 0.005),Point B (0.4, 0.25, 0.35), Point C (0.52, 0.01, 0.47), Point D (0.9,0.005, 0.095), and Point E (0.9, 0.09, 0.01) or on the lines connectingthose Points.
 21. The glass composition according to claim 20, whereinthe rare earth element is at least one element selected from the groupconsisting of lanthanum and neodymium.
 22. The glass compositionaccording to claim 20, being a crystallized glass or an amorphous glass.23. The ceramic substrate composition according to claim 2, furthercomprising at least one of (a) at least one oxide selected from thegroup consisting of aluminum oxide and titanium oxide in an amount of50% or less by weight of the total amount, (b) a mixed oxide ceramicmaterial including at least one crystal phase selected from the groupconsisting of ReBO₃, Re₃BO₆, Re(B₂O₃)₃ and ReB₃O₆, (c) a crystallizationglass that can precipitate at least one crystal phase selected from thegroup consisting of ReBO₃, Re₃BO₆, Re(B₂O₃)₂ and ReB₃O₆ and (d) acrystallization glass that can precipitate at least one crystal phaseselected from the group consisting of ReBO₃, Re₃BO₆, Re(B₂O₃)₂ andReB₃O₆.
 24. The ceramic substrate composition according to claim 24,wherein the mixed oxide ceramic material is 20% to 90% by weight of thetotal amount of the glass and the mixed oxide ceramic material, iscalcined dB₂O₃-eRe₂O₃ in which the molar ratio d/e=1/3 to 3/1 andoptionally further contains ZnO; and wherein the crystallization glassis is 20% to 90% by weight of the total amount of the glass and thecrystallization glass and is represented by gB₂O₃-hRe₂O₃-iZnO in whichg+h+i=1 and the molar amounts g, h, and i fall within a region definedin a ternary composition diagram by Point F (0.4, 0.595, 0.005), Point G(0.4, 0.25, 0.35), Point H (0.52, 0.01, 0.47), Point I (0.9, 0.005,0.095), and Point J (0.9, 0.09, 0.01) or on the lines connected thosePoints; and wherein the total molar amounts of B₂O₃, Re₂O₃ and ZnO inthe composition are such that they fall within a region defined in aternary composition diagram by Points A through E or on the linesconnected those Points.